1. Field of the Invention
The present invention relates to a method for printing a bonding agent such as a paste solder to lands formed on a printed circuit board in order to bond chip components such as chip condensers to the lands.
2. Description of the Related Art
Generally, chip components such as chip capacitors, chip resistors, etc., are bonded on a printed circuit boards for use in a computer and other devices employing solid state circuitry. To bond those chip components on the printed circuit board, first, paste solder is printed on lands 21 formed on the printed circuit board, as shown in FIG. 9, via a printing screen or mask. The paste solder is typically made by combining grained solders and flux. The chip components are placed on the solder layer printed on the lands, and the solder is molten by reflow soldering, a process wherein the board is heated to make the solder flow. Next, the molten solder is applied to individual chip components to be bonded. Solder fillets 22, as shown in FIG. 9, are formed at the distal portions of the chip components 20 and secure the components 20 to the corresponding lands 21.
However, when the chip components are deposited on the lands 21 containing more than adequate amount of paste solder, the paste solder has a tendency to cover the distal ends of the chip components as illustrated in FIG. 10. Such mishapped solder fillets 22 are particularly susceptible to the formation of cracks due to the difference of thermal expansion coefficients between the printed circuit board and the chip components. With the introduction of cracks in the solder fillets 22, the electrical connection between the soldered components on the printed circuit board may degrade to the point where there is insufficient electrical connection between chip components. This, in turn, results in a decrease in the overall reliability of the printed circuit board. To solve this problem, the amount of solder printed on the lands 21 should be properly adjusted to form and shape solder fillets 22 having the shape shown in FIG. 9.
To maintain proper adjustment to the amount of printed solder, various solder printing methods have been proposed which take into account conditions which often vary from one printing application to the next, such as, printing pressure and mask thickness. However, according to these methods, the solder amount adjustment consists in regulating a single amount of solder formed into fillets 22 for all the lands in a single printing application. Such methods are not designed to account for the amount of solder applied to individual or specific lands. Therefore, the amount of solder to be applied in a particular application is not adjustable with respect to individual chip components having varying heights from the surface of the printed circuit board. As a result, improperly shaped solder fillets can not be avoided.
One proposed solution to the above-described drawback has been to use a method which adjusts the amount of solder applied to every land by partially reducing the outer diameter of each mask opening by a degree smaller than that presented by the relevant portion of the corresponding land.
However, according to this proposed method, the formation of well shaped solder fillets still cannot always be achieved. The reason for this is that an oxide layer, typically formed on the surface of the land, inhibits the flow of molten solder to only those areas of the land from which the oxide layer has been removed. This restriction, in effect, increases the chances of the molten solder fillets 22 covering the individual chip components, since the molten solder is confined to only those specific areas of the land which have been deoxized, i.e., those portions which will contain printed patterns. When the molten solder is so confined to wetting on the deoxidized area only, the chances are significant that the solder fillet 22 will overlap the chip components. This is due, once again, to the fact that the molten solder is confined by the oxide coating on the land to only those land portions which have been deoxidized by the action of the flux. Consequently, this method suffers from the drawback that solder fillets 22 often can not be formed without covering a portion of the chip components.
Moreover, a further disadvantage of reducing the size of the mask patterns for purposes of maximizing the efficiency of solder deposit occurs due to the frequent mislocation of chip components placed on the printed circuit borard. With the decrease in size of the masking patterns, the amount of solder to be deposited becomes likewise reduced. When, as it is inevitably the case, chip components are placed on the circuit board in a slightly dislocated position, one end of the chip may be soldered in place with a greater amount of solder than the other end of the same chip. When the solder solidifies, the chip component frequently becomes unballanced due to the differing rates of encasement of both chip ends in the solidifying solder. This condition frequently results in the electrical isolation of one end of the chip from the other.